Capacitive sensing is a technology based on capacitive coupling which takes human body capacitance as input. The capacitive touch sensor has been widely used in smart phones, tablets and even in the IT displays up to 23 inches, e.g. Notebooks, laptop trackpads, digital audio players, computer displays, ALL-in-one PCs, with the multi-touch features.
More and more design engineers are selecting capacitive sensors for their versatility, reliability and robustness, unique human-device interface and cost reduction over mechanical switches.
Capacitive sensors detect anything that is conductive or has a dielectric different than that of air. While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch and gesture-based touchscreens are also premised on capacitive sensing.
Capacitive sensors are constructed from many different media, such as copper, Indium Tin Oxide (ITO) and printed ink. Copper capacitive sensors can be implemented on Printing Circuit Boards (PCBs) as well as on flexible material. Indium Tin Oxide allows the capacitive sensor to be up to 90% transparent for one layer solutions, such as touch phone screens.
There are two types of capacitive sensing system: mutual capacitance, where the object (finger, conductive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially; and self- or absolute capacitance where the object (such as a finger) loads the sensor or increases the parasitic capacitance to ground. In both cases, the difference of a preceding absolute position from the present absolute position yields the relative motion of the object or finger during that time.
FIGS. 1A and 1B show the structures of the traditional two-dimensional sensor arrays (1010, 1020). To have better coordination accuracy of the touched locations, the touch sensors often come with two-dimensional sensor arrays, including Double-sided Indium Tin Oxide (DITO) or Single-sided Indium Tin Oxide (SITO). The size of the sensor element from the sensor array is about the finger size (5-8 mm). The patterns of the sensor elements are mostly as the bar shape, the diamond shape or other polygon shape. For example, FIG. 1A shows that the pattern of the sensor elements (1018, 1016) in a two-dimensional sensor array 1010 is the bar shape and the two-dimensional sensor array 1010 includes a bottom layer 1012 and a top layer 1014, and FIG. 1B shows that the pattern of the sensor element 1022 in a two-dimensional sensor array 1020 is the diamond shape.
By referring to FIG. 1B, the connecting line “Xm” attaches to the m-th electrode in the horizontal axis, and the connecting line “Yn” attaches to the n-th electrode in the logitudinal axis. Thus, the trace routing for the two-dimensional sensor array 1020 whose number of traces is the number of electrodes in the horizontal axis plus the number of electrodes in the longitudinal axis, i.e., m+n, is easier than the one-dimensional sensor array.
FIGS. 2A and 2B show the structures of the traditional one-dimensional sensor arrays (2010, 2020). As the cost is concerned, especially the touch panel module takes a certain amount of total system cost, the one-dimensional sensor array came up, however, with the compromise of lower coordination accuracy. In order to have the multi-touch features on the one-dimensional sensor, the pattern design of sensor element becomes crucial. For example, FIG. 2A shows that the pattern of the sensor elements 2012 in a one-dimensional sensor array 2010 is the triangle shape, and FIG. 2B shows that the pattern of the sensor elements 2022 in a one-dimensional sensor array 2020 is the saw-tooth shape.
The sensor elements should be normally small while maintaining the touch accuracy or the resolution. This makes the trace routing difficult for the individual sensor element under the defined active area of the touch sensor. For example, FIG. 2B illustrates that the trace routing of the circuit 2024 for the individual sensor element 2022 is difficult under the defined active area of the one-dimensional sensor array 2020.
In general, the two-dimensional sensor array constructed as a matrix-like or keyboard-like structure has less constraint on the trace routing and provides much better touch accuracy comparing to the one-dimensional sensor array for multi-touch applications. The major drawback is the high cost in the manufacture.
On the other hand, one-dimensional sensor array is bounded by the routing space providing barely satisfied touch accuracy, but with the advantage from the cost. Under the limitation of touch accuracy, the size of one-dimensional sensor array for multi-touch is limited under 5 inches.
Currently, the capacitive touch panel with the sensor elements composed of a single material layer transmits signals from each sensor element by a separate connecting line, and determines the occurrence of the touch on the basis of the change of the singles from each sensor element directly. Thus, although the fabrication cost and working hours are reduced, it requires much more connecting lines to achieve the sensing accuracy, and results in difficulties on the design of wiring and connecting interface. On the other hand, when reducing the number of connecting lines, it will reduce the number of sensor elements and thus the sensing accuracy. Therefore, it is desirable to create a sensor array to resolve the above-mentioned issues.